Add PointerType analysis for DirectX backend

As implemented this patch assumes that Typed pointer support remains in
the llvm::PointerType class, however this could be modified to use a
different subclass of llvm::Type that could be disallowed from use in
other contexts.

This does not rely on inserting typed pointers into the Module, it just
uses the llvm::PointerType class to track and unique types.

Fixes #54918

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D122268